A PTC-element, also called a PTC-thermistor, has a positive temperature coefficient of electrical resistance, wherein the electrical resistance thereof increases as the temperature is elevated, and is used as a protective device for protecting against overcurrent, overvoltage and other phenomena involving heat evolution. Currently used PTC-elements are is composed of a matrix of a non-conductive crystalline polymer and dispersed therein, an electroconductive particulate material. The electrical resistance of such a PTC-element is low at lower temperatures since the particles of the conductor material are held in contact with each other at lower temperatures. However, when the temperature is elevated, the electric resistance increases by the possible occurrence of the separation of the particles of the conductor from their contact with each other due to the thermal expansion of the non-conductive crystalline polymer and reaches finally a current shutoff.
As PTC-elements of the prior art, there have been proposed, for example, those in which spherical carbon particles, in particular, a perfect spherical shape, are used for the conductor material (Japanese Patent Application Laid-open Hei-8-298201 A) and those in which the spherical carbon particles above are each provided with a surface coating layer of metal (Japanese Patent Application Laid-open Hei-8-172001 A).
Those PTC-elements using a spherical particulate conductor material are said to have a low volume resistivity, a high electrical resistance at the peak and a capability of a steep increase in resistance within a narrow range of temperature.
Upon mounting such a conventional PTC-element on the surface of an electric circuit board, however, a problem is brought about in which the initial volume resistivity of the PTC-element is, in some cases, set at a high value due to a possible temperature elevation upon the assemblage thereof, when a high temperature processing, such as a reflowing treatment of the solder, is employed, whereby its use as an assembled PTC-element becomes difficult.